Colloidal semiconductor and method of manufacture

ABSTRACT

A photoemissive cathode is provided consisting of colloidal semiconducting material on a base transparent to visible and/or infrared radiation. The semiconductor material may be a binary or ternary compound from the groups III-V or II-VI of the periodic table. The semiconducting colloids are formed by a simultaneous precipitation and doping process.

States Patent 1191 [111 3,925,698

1451 Dec.9, 1975 Stahl [54] COLLOIDAL SEMICONDUCTOR AND 3,295,002 12/1966 Amans 252/500 x METHOD O MANUFACTURE 3,562,003 2/1971 Frohlich... 252/500 3,575,628 4/1971 Word 250/213 VT Inventor: Herbert Slahl, sprmgfield, 3,625,869 12/1971 Marks 252/500 [73] Assignee: The United States of America as 3,634,692 l/l972 Padovani 250/213 V":

ted b the Secret of the 3,672,992 6/1972 Schaefer 313/9 P a g g D C Y 3,696,262 10/1972 Antypas 313/102 [22] Filed: Oct. 12, 1973 Primary Examiner-Saxfield Chatmon, Jr.

Attorney, Agent, or Firm-John E. Holford; Nathan [21] Appl' 406048 Edelberg; Robert P. Gibson [52] US. Cl. 313/346; 313/94; 2255220550108; [57] ABSTRACT 51 1m. 01. 1101.1 1/14; HOlJ 19/06 A Photoemissive Cathode is Provided Consisting of [58] Field 61 Search 313/94, 101, 102,346; loidal semiconducting material a base transparent 250/213 357/2, 5 5 518 to visible and/or infrared radiation. The semiconductor material may be a binary or ternary compound [56] References Cited from the groups III-V or lI- Vl of the periodic table. UNITED STATES PATENTS The semiconducting colloids are formed by a simultaneous precipitation and dopin g'process. 2,877,371 3/1959 Orthuber et al. 252/500 3,251,714 5/1966 Rotschild 252/500 x 4 Claims, 7 Drawing Figures US. Patent Dec. 9, 1975 Sheet 1 of 3 3,925,698

L- sr-m="r-wl I M l l M GOLD M I COLLOIDAL GOLD z FOIL m RUBY GLASS LU z D g U. E o 2 0 Z 9 O z x LU WAVELENGTH FIG.

z 1 g COLLO|DAL SODIUM PARTICLES IN 'ROCK SALT LL] 0 O 2 9 D. D: o

(I) M '3 SODIUM METAL UNIFORMLY EMBEDDED m ROCK SALT o l I WAVELENGTH US. Patent Dec. 9, 1975 CONDUCTION BAND sheet 2 on 3,925,698

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VALENCE BAND i FIG. 3;:

FIG. 3b

FIG. 50

US. Patent Dec. '9, 1975 Sheet 3 of3 3,925,698

COLLOIDAL SEMICONDUCTOR AND METHOD OF MANUFACTURE The invention described herein may be manufactured, used, and licensed by or for the Government for governmental purposes without the payment to me of any royalty thereon.

BACKGROUND OF INVENTION There has been widespread interest in recent years directed to optical survelliance and detection devices based on wideband response to frequencies both within and outside the visible spectrum. Many operations in the medical, law enforcement, commercial processing, equipment maintenance and military fields require real time devices of this type. Solid state devices which comprise light sensitive surfaces such as photoconductors, or photoemitters are now used almost exclusively to meet the real-time requirement, and the wide band requirement as well. The most successful devices to date employ monocrystalline material consisting of elements from the third and fifth groups of the periodic system which are doped with an element from the second or first group of the periodic system of the elements. Amorphous materials also demonstrate semiconductor properties, but because they are heavily disordered, the mathematical approaches used with monocrystalline and polycrystalline materials are inadequate to predict the phenomena encountered. In addition, when semiconductors are fabricated as thin films for photoelectronic cathodes in multipliers, image converters, shutter or storage tubes, orthicons, vidicons and X-ray intensifiers, the mathematics which apply to bulk materials are even less applicable. As a result, little is known about semiconductors with extremely high surface to volume ratios in general, and colloidal film semiconductors in particular.

SUMMARY OF THE INVENTION The object of the present invention is to utilizethe unique properties of colloidal semiconductors to enhance the quantum efficiency of a photocathode at low optical frequencies. The size of the colloidal particles is of prime importance in determining the characteristics of the photocathode, so that'one aspect of the present invention involves the method of forming these particles.

BRIEF DESCRIPTION OF THE DRAWINGS The above and other objects of the invention will'be best understood with reference to accompanying drawings wherein:

FIG. 1 shows the extinction coefficient for a thin film of metallic gold versus the same coefficient for colloidal gold suspended in ruby glass.

FIG. 2 shows the absorption coefficient of large particles of sodium in rock salt versus colloidal sodium in the same medium;

FIG. 3a shows the band structure of a typical p-type semiconductor crystal illustrating the band bending at the surface; I

FIG. 3b through 3d show the band structure of a semiconductor with progressively smaller sizes of the crystal; and I FIG. 4 shows a deposited photocathode just prior to the removal of the precipitating solution.

Referring to FIG. 1 the fundamental difference between colloidal and bulk material for optical purposes is shown using the metal gold. When the metal is reduced to a transparent foil sufficiently thin to pass white light the resulting transmitted light is distinctly green. When this same material takes the form of a colloidal suspension in a suitable host material, such as corundum or glass, the resulting transmission is distinctly red (ruby, ruby glass). This shift to longer wavelengths or less energetic transmitted photons is also characteristic of silver.

FIG. 2 evidences that a similar shift is realized when a crystalline material such as rock salt is used for the host material. The absorption coefficient of the metal sodium is shown in this plot. The case of color centers refers to sodium particles exhibiting a spectral absorption curve distinctly displaced toward infrared against the beforesaid curve.

Various degrees of shift similar to those described above can be demonstrated in the spectral absorption coefficient of a deposited metal film by varying the rate of evaporation, and thereby influencing the size of the condensed particles.

In general, systems containing dispersed solid colloidals are known for their enormous surface activity which, in turn, results in great catalytic power, and in enhanced adsorptive properties. Essential are the enormous surfaces which characterize colloidal matter. For example, a cube of silver one centimeter on a side with a surface area of 6 square centimeters provides a surface of 600,000 square centimeters or 50 square meters when subdivided into 10 tiny cubes 0.1 micron on a side. For millimicron cubes the surface area becomes as big as 6000,000 square meters corresponding to a huge football field. The increase in surface area is accompanied by a proportional increase in unsaturated or dangling ionic bonds. It is apparently this fact that accounts for the shift of the optical absorption (or extinction) coefficient maxima towards longer wavelengths, as well as all adsorptive, and catalytic peculiarities of colloidal matter.

Referring to FIG. 3a, in the case of crystalline semiconductors'the effect of unsaturated bonds at the surface is best explained by a band model. The material ought to have a moderately wide band gap, and low electron affinity. For p-type semiconductors there is an appreciable downward bending of the conduction, valence and eventual intermediate bands caused by surface states.

Even for heavily doped (but still non-degenerate) material this bending penetrates only a short distance into the interior of the crystal, usually in the vicinity of -200 Angstrom units (l0-20 millimicrons, or 0.01 to 0.02 microns). The bending effect also lowers the vacuum level against the valence band in the bulk where most photoelectric electrons originate. As a result, the escape probability of theexcited electrons is enhanced for p-type semiconductors as compared to plain material.

It should be also mentioned that excited electrons can loose energy on their way to the surface so that each material is characterized by a specific, averaged, escape depth. Analogously, an averaged absorption depth of the incident photons can be defined. lt is obvious that an effective photoelectric emitter must imply an escape depth much larger than the absorption depth. I

FIG. 3a shows the band model bending effect that occurs in all large semiconductors normally within 100 200 angstroms of the surface. FIG. 3b shows the band model for a colloidal particle having dimensions of 100 millimicrons. Bending is shown at both of the opposed surfaces while the flat portion represents the band boundaries for the bulk inner portion of the crystal particle. The average level of the band gap is nearly that of the flat inner portion, but the surface bending is beginning to reduce it noticeably. FIG. 30 shows the band boundaries for a 50 millimicron particle where the effect is still more pronounced. Finally FIG. 3d shows the same curves for a particle less than 50 millimicrons where the average band gap levels are reduced everywhere inside the particle. As FIGS. 30 and 3d evidence, this is the more pronounced the smaller the size of the particle. The ratio of the overall volume of the surface layer (as defined above) to that of the compact portion of the crystal grows tremendously the smaller the size of the individual crystal. This is synonymous with the fact that, in colloidal matter, the surface plays a dominating role as far as physical and chemical properties of the material are concerned. At the highest degree of dispersion, the unsaturated, dangling surface bonds determine the physical and chemical properties of the material because the role of the bulk portion is nil.

As a result, colloidal matter can be be explained in terms of the notions and laws developed so far for the solid state. The aforementioned shifts of the absorption and extinction spectra towards infrared indicate that less energetic quanta of light are capable of interactions with colloidal lattice material, i.e. also of liberating electrons.

It is known that photoelectric emission excited by absorbed long-wavelength, near threshold, radiation originates from the topmost regions of the emitting material. The larger the portion of the superficial matter in a given material is, the more pronounced the near-thre shold emission, usually denoted as tail emission, will be. This is corroborated by the fact that semiconducting, especially fluffy photocathodes of the silver-oxygen-cesium type exhibit a distinctly enhanced emission in the near infrared. Not only the familiar S-l emitter shows this phenomenon, but also germanium emitters which feature copper, gold, or palladium as base material. The same situation is true for the commercially used S-3 photoelectric cathode that consists of colloidal silver, oxygen and rubidium. All these emissive layers are typified by an appreciable surface-to-bulk ratio as compared to a fully compact photoelectric emitter.

According to the present invention this enhanced near-threshold emission can be maximized in other semiconducting compounds which are characterized by much greater quantum efficiencies (perhaps by more than an order of magnitude) in the visible and near infrared frequency range. Such compounds are obtained by combining elements from the third and fifth group or the second and sixth group of the periodic table. The metallic elements from group III particularly suitable for photoelectric cathodes are gallium, indium, and eventually also aluminum and thallium. They readily form phosphides, arsenides, antimonides, or bismuthides. Analogously, metals from the second group of the periodic system namely beryllium (glucinum), magnesium, calcium, strontium or barium readily form oxides, sulfides, selenides or tellurides. All the compounds may be binary, ternary or quaternary in nature. Each compound will include at least one metallie, and one non-metallic element with additional elements of either type added as desired. As previously stated, the most important feature of the materials according to the present invention is that the compound is produced in form of individual microcrystallites of colloidal size, and employed as the base material of the photoelectric cathode (as the first layer embraced in the production of said emitter). Also included in the present invention are elements in colloidal form from the forth group of the periodic system which are by nature iso-electronic with all the beforesaid compounds.

The usual methods of manufacture of colloidal matter such as vapor deposition, microgrinding in colloid mills, or dispersion by means of ultrasound or underwater spark are not suitable for colloidal semiconductors according to the present invention. These methods either distort the lattice structure of the semiconductor, evaporate at least one of the elements in the compound or produce a continuous film rather than individual colloidal particles. The method preferred here is to precipitate the reaction product from a suitable solution at room temperature.

FIG. 4 shows the photocathode after precipitation. The precipitation is preferably done in a clean container 41. The precipitate is either collected on the bottom of the container, or a portion of its deposits directly on base plates, such as plate 42, which are to be employed as the windows of the completed devices. In the first case, the precipitate essentially a colloidal powderis collected, washed, dried, and stored for subsequent sedimentation onto window plates. In the latter case, the individual window plates are placed onto the bottom of the container. Preferably, they are round, approximately 1 2 inches in diameter and oneeighth to one-fourth inches thick, and are made from glass, quartz or other crystals transparent to the light, or radiation, to be employed with the finished optoelectronic device. The container ought to exhibit the same spectral transmission as the aforesaid windows. The thickness of the deposited layer can readily be monitored by a light beam 43 that traverses one of the plates, and the bottom of the container referred to. As the precipitate 44 collects on the substrate plates, the intensity of the emerging light will decrease. When the level reaches a pre-determined percentage of the initial intensity, the solution 45 is gently syphoned off. The substrate plates are then removed from the container, and allowed to dry in a dust-free cabinet or room. Afterward, these base plates are to be sealed to the envelopes of image converters, or other opto-electr-ohic devices, and subjected to the professional fabricating steps of such devices.

According to the present invention, the solvents employed for the production of colloidal llIV compounds are viscous water-glycerine, water glycol, water-sugar, or other congenial mixtures whose viscosity at room temperature can be controlled by the concentration of the additive. The sugar is not restricted to the well known, commercially available kinds such as destrose, fructose, saccharose or invert sugar but instead all sugars in the widest sense of chemistry may be embraced reducer may be arsine (AsI-I phosphine (PI-I stibnine (SbH or bismuth hydride (Bil-I All reactions conceivable can be summarized under the denomination Oxidation-reduction (Redox) reaction. A partial list of suitable solutes include:

Gallium bromide Indium bromide chloride chloride iodide iodide chlorate flouride nitrate chlorate oxalate nitrate selenate selenate sulfate sulfate Similar listings can also be established for aluminum and thallium. By using one or two additional partners from the pertaining group, ternary or quaternary compounds can be formed in the same precipitation process. For the productionof colloidal II IV compounds according to the present invention, a variety of processes is available.

The present invention embraces also the doping of the colloidal semiconducting crystallites during the formation process. Compounds of elements from the third and fifth group of the periodic table are usually doped with elements from the first or second group of the periodic system in order to produce p-type semiconduc tors. Understandable, the dopant-compound is to be employed in low concentrations only. Examples of suitable compounds for doping are:

Zinc acetate Beryllium bromide bromide chloride chloride flouride chlorate iodite perchlorate nitrate formate oxalate nitrate orthophosphate permanganate picrate selenate sulfate Analogously copper, silver or gold compounds can also be employed for doping of colloidal III-V type semiconductors. In order to dope colloidal materials which consist of elements from the II and VI group of the periodic system a relevant list of compounds reducible by the afore-mentioned reducers, is available. According to the present invention, the precipitation of the host semiconductor ensues simultaneously with that of the dopant so that the latter is embedded as a trace element in the former material.

I claim:

1. A photocathode comprising:

a transparent window plate; and a base layer on said plate of photoemissively active semiconductor material consisting of individual, colloidal particles deposited by the evaporation of their suspension medium, said particles being under millimicrons in size and formed of compounds from a group consisting of elements from columns III and V, or II and VI, of the periodic system of elements containing a p-type dopant.

2 A photocathode according to claim 1 wherein:

said colloidal semiconductor material is doped with an additional element from Groups I through III of the periodic system.

3. A photocathode comprising:

a transparent window plate; and a base layer deposited on said plate of photoemissively active semiconductor material consisting of individual, colloidal particles deposited by the evaporation of their suspension medium, said particles being under 100 millimicrons in size and formed of elements from column IV of the periodic system of elements containing a p-type dopant.

4. A photocathode according to claim 3 wherein:

said dopant is an element from Groups I through III of the periodic system. 

1. A PHOTOCATHODE COMPRISING: A TRANSPARENT WINDOW PLATE; AND A BASE LAYER ON SAID PLATE OF PHOTOEMISSIELY ACTIVE SEMICONDUCTOR MATERIAL CONSISTING OF INDIVIDUAL, COLLOIDIAL PARTICLES DEPOSITED BY THE EVAPORATION OF THEIR SUSPENSION MEDIUM, SAID PARTICLES BEING UNDER 100 MILLIMICRONS IN SIZE AND FORMED OF COMPOUNDS FROM A GROUP CONSISTING OF ELEMENTS FROM COLUMNS III AND V, OR II AND VI, OF THE PERIODIC SYSTEM OF ELEMENTS CONTAINING A P-TYPE DOPANT.
 2. A photocathode according to claim 1 wherein: said colloidal semiconductor material is doped with an additional element from Groups I through III of the periodic system.
 3. A photocathode comprising: a transparent window plate; and a base layer deposited on said plate of photoemissively active semiconductor material consisting of individual, colloidal particles deposited by the evaporation of their suspension medium, said particles being under 100 millimicrons in size and formed of elements from column IV of the periodic system of elements containing a p-type dopant.
 4. A photocathode according to claim 3 wherein: said dopant is an element from Groups I through III of the periodic system. 